A radiator fan motor (hereinafter simply called a “motor”) installed in a vehicle is taken as an example of a motor load. FIG. 10 is a circuit diagram showing a driving circuit of a conventional motor M1. As shown in FIG. 10, the driving circuit includes an engine control unit (hereinafter called an “ECU”) 101, a battery VB, and the motor M1. Then, a relay X1 is arranged between the battery VB and the motor M1 and further, a relay X2 is arranged between the motor M1 and a ground. In addition, numeral “VB” shall hereinafter be used in both of the case of showing the battery itself and the case of showing a voltage output from the battery.
An output terminal A of the ECU 101 is connected to an exciting coil of the relay X1, and an output terminal B of the ECU 101 is connected to an exciting coil of the relay X2.
The relay X2 has transfer contacts and when connection between normally-closed contacts c and b is made, one end of the motor M1 is grounded to the ground through a resistor R101. When connection between normally-open contacts c and a is made, one end of the motor M1 is directly grounded to the ground.
Then, when a cooling water temperature is relatively low and it is necessary to rotate a fan, the ECU 101 energizes an output signal S1 of the terminal A to turn on the relay X1 and shuts off an output signal S2 of the terminal B to turn off the relay X2. Consequently, the battery voltage VB is applied to a series circuit of the motor M1 and the resistor R101 and the voltage applied to the motor M1 decreases by a voltage drop occurring in the resistor R101, so that the motor M1 rotates at low speed.
On the other hand, when the cooling water temperature becomes high, the ECU 101 energizes the output signal S2 of the terminal B to turn on the relay X2. Consequently, the normally-open contacts c and a of the relay X2 conduct and the battery voltage VB is applied to the motor M1 and the motor M1 rotates fully.
Here, the motor M1 locks when rotation of a radiator fan is forcibly blocked by submersion etc. of the radiator fan or snow engages with the radiator fan during travel of the vehicle. Then, a lock current flows in the motor driving circuit, and the motor M1 and the driving circuit generate heat.
Here, a PTC element (a resistive element with a positive temperature coefficient) is normally incorporated into the motor M1 in series with an armature. When a temperature of the motor M1 is low, the PTC element indicates a low resistance value and does not block a motor driving current, but when a large current like the lock current continues to flow, a temperature of the PTC element increases by Joule heat and the motor driving current is limited and heat generation is prevented. Also, rotation of the motor M1 returns to the original state when a factor in blocking the rotation of the radiator fan is eliminated.